In a steam turbine, steam is expanded to generate rotational energy. The steam turbine has a plurality of stages, each stage having a guide vane ring with a plurality of guide vanes and a rotor blade ring with a plurality of rotor blades. The rotor blades are fitted to the shaft of the steam turbine and rotate during operation of the steam turbine; the guide vanes are fitted to the housing of the steam turbine and are stationary.
During operation of the steam turbine, the blades or vanes are exposed to various degradation processes, for example creep. Creep is slowly proceeding plastic deformation of the blade or vane under the action of a load at an elevated temperature. The load can originate, for example, from the steam flowing in the steam turbine and, in the case of the rotor blades, can be caused by the centrifugal force on account of their rotation. Furthermore, the surfaces of the blades or vanes can oxidize at appropriately high temperatures in the presence of steam, high temperature levels of this type regularly being achieved in a high-pressure steam turbine or in a medium-pressure steam turbine and typically being more than 540° C.
In the case that an oxide layer has formed on the surfaces of the rotor blades and/or of the guide vanes, micro-notches or micro-cracks can form on the surfaces. This reduces the strength of the rotor blades and/or of the guide vanes, and therefore, for example, the bending fatigue strength of the rotor blades and/or of the guide vanes is reduced or at least influenced.
To avoid the oxidation, the rotor blades and/or the guide vanes are conventionally produced from high-cost alloys.
US 2009/123737 A1 discloses turbomachine components having a main body which comprises chromium steel. The main body is coated with a nitrided hard layer. A layer comprising chromium nitride is applied to the latter by vapor deposition.
DE 10 2008 020 607 A1 discloses an object with a first surface and a coating applied thereto. The coating comprises a first layer comprising chromium nitride, to which a further layer is applied.
DE 10 2008 019 891 A1 discloses an erosion-resistant coating in particular for gas turbine components. The coating in this case has alternately relatively hard and relatively soft layers. For better adhesion, an adhesion promoting layer comprising chromium nitride is provided directly on the gas turbine component.
EP 2 230 330 A1 discloses a plastics component having an erosion-resistant layer for applications with erosive loading. The erosion-resistant coating is a multilayer coating system with one layer consisting of a metal and one layer consisting of ceramic, or ceramic layers of differing hardness.